Polymer materials are ubiquitous in everyday life and are used in various applications (such as, medicals, automobiles, electronics, structures, etc.). These materials experience stress during the normal use, which can lead to damage and failure of the product. Having the ability of detecting damage and locating areas under high stress in situ is essential to eliminating failure of the polymer materials.
Mechano-responsive luminescent materials change the colour of their luminescence isothermally, and because these changes can be easily detected, they have potential use for practical applications such as mechano-sensors, indicators of mechano-history, security papers, opto-electronic devices and data storage. For example, deformation, distortion and destruction of various materials can be detected more easily through mechanochromic luminescent materials than using traditional methods. These sensing properties can also be useful for the maintenance of materials because they can easily show where the damage has occurred.
The simplest manner to provide a mechanochromic material is to incorporate a coloured substance into the matrix in the form of capsules or hollow fibers, as described in WO2007/003883A1. Initially, the colour is not visible. Upon damage to the matrix, the capsules or fibers rupture and expose the coloured substance, such as a fluid or solid. WO2006/105290A2 discloses a two-part system, wherein a colourless compound is mixed with an activator. Upon the rupture of their respective containers, a colour change is trigged. A disadvantage of these systems is that the fibers and capsules need to be evenly dispersed throughout the matrix, so that the damage inducing force has a large chance to intersect the capsules or fibers.
U.S. Pat. No. 7,242,443B2 discloses another approach, wherein triboluminescent materials are used to give off flashes of light in response to stress or damage. When damage occurs, these materials require continuous monitoring to detect due to the transient nature of the light flash.
U.S. Pat. No. 7,244,500B2 discloses smart coatings consisting of several layers of sensing materials. These coatings are complex and require external power to accomplish many of their tasks.
U.S. Pat. No. 8,236,914B2 discloses a self-assessing mechanochromic material, which is a mechanochromic material including a polymer having a backbone containing a mechanophore.
Nallicheri, et al. (“Investigations of the Mechanochromic Behavior of Poly(urethane-diacetylene) Segmented Copolymers”, Macromolecules, 1991, pp. 517-525, Vol. 24, No. 2) discloses a diacetylene segmented copolymer, which exhibits a shift of colour when subjected to a strain.
Todres (“Recent advances in the study of mechanochromic transitions of organic compounds”, J. of Chemical Research, 2004, 89-93) outlines several organic compounds possessing mechanochromic properties. Specifically, spiropyran has been noted to undergo a colour change upon grinding. However, few applications exist for the small molecule alone. Weder and coworkers (Weder, C. Mechanochemistry: Polymers react to stress. Nature 459, 45-46 (2009)) have incorporated cyano-substituted oligo(p-phenylene vinylene) derivatives into different polymer matrixes and have synthesized “self-assessing” polyurethanes, polyethylene blends, poly(ethylene terephthalate)s, and poly(ethylene terephthalate glycol)s. Their approach relies on the initial formation of nanoscale aggregates of the sensor molecules in the polymer matrix. Upon deformation, the cyano-substituted oligo(p-phenylene vinylene) sensors are transformed from excimer to monomer and a shift in the emission spectrum is observed. Most of these sensing units are not chemically incorporated into the backbone, and exhibit only a fluorescent colour change that is not visible to the naked eye. Additionally, these materials are not reversible in colour change and can only exhibit a colour change once.
Kim and Reneker (“A mechanochromic smart material”, Polymer Bulletin, 31, 1993, 367-374) introduced azobenzene into a copolyamide oligomer, which was chemically incorporated into a polyurethane. Upon exposing the material to tensile stress, a change in the UV spectrum at 375 nm was observed. However, no visible change was noted and the polymer had to be irradiated with UV light prior to stressing the materials.
In the recent patent document WO 2009018111 A1, David and his coworkers presented a mechanochromic material comprising a polymer having a backbone containing a mechanophore, which is used as an additive to get colour change under mechano-force.
The number of mechano-responsive luminescent materials based on molecular assemblies is still limited, compared with that of dynamic luminescent materials responding to heat or light.
In most of the reported mechanochromic materials, an additive having a mechanochromic property, such as, a dye or a luminescence agent, was used as luminescence resource in the polymer matrix.
The problem to be solved by the present invention is to provide a mechanochromic material which has mechanochromic properties without the need of an additional mechanochromic component, such as a dye or a luminescence agent. It should be a simple system with only a few components so that the production and use are simplified. Additionally, it would be beneficial that the response to mechanical stimuli could be observed within the visible light spectrum. Preferably, the mechanochromic material should be able to recover so that its mechanochromic response is reversible and the material can be used longer and indicates several mechanical stimuli.